A mass data storage system typically provides redundant data storage in a mirrored form to guard against the possibility of loss of data due to a failure in the system. The existence of multiple copies of the data not only protects against loss of the data, but affords an opportunity to use the multiple copies to improve the performance of the system, compared to systems having only one copy. The possibility for improvement is based on the fact that providing multiple copies may, of itself, enable the system to choose which of a particular copy is to be accessed.
Methods are known in the art for implementing such choice. U.S. Pat. No. 6,112,257 to Mason, Jr. et al., which is incorporated herein by reference, describes a method for dynamically adjusting a policy for accessing a mass storage system having mirrored volumes. Statistics of reading and writing data to/from the volumes are collected over a period of time. Depending on the statistics, the policy is maintained or is changed. The policy determines which of the mirrored logical volumes is accessed to service a read or write request.
U.S. Pat. No. 6,611,896 to Mason, Jr. et al., which is incorporated herein by reference, describes a method for adjusting a seek policy for mirrored and non-mirrored physical devices in a storage system. Statistics for accessing mirrored logical volumes on the physical devices are collected. The statistics are used to determine and minimize activity levels for non-mirrored physical devices.
The Storage Performance Council, of Redwood City, Calif., maintain a web site at http://www.storageperformance.org. The council provide benchmarks for characterizing storage systems, including a least response time (LRT™) measurement that characterizes the time for a storage system, operating at a low throughput rate, to respond.